Critical strike

A critical strike is a modifier that causes an attack to deal, typically, twice its normal damage. This could represent an attack that hits harder, a lucky strike or damaging a weak area on an opponent.

Most champions have different attack animations for a critical strike, though some do not; this includes mages and other champions not meant to be built using basic attacks for damage. Some of these champions performing a critical strike (if built being able to perform them) may instead have a soundbite that plays in place of an animation, typically a grunt of effort.

Players can increase the chance and the damage of critical strikes with items, abilities, and runes.

Critical strike is valuable for all champions who primarily deal damage with automatic attacks, rather than through abilities. Some champions, such as Ashe, have an affinity for critical hits and benefit more from items that improve them.

It's important to note that you cannot score a critical strike against a building such as a turret or inhibitor.

When averaged over a large number of autoattacks and at base critical damage, critical strike chance is essentially a damage multiplier where each % of crit chance adds 1% damage. Thus at 50% critical strike chance a champion will be doing (on average) 150% damage with autoattacks, not counting bonus critical strike damage. Because of this, critical strike chance scales well with both attack damage and attack speed. The formula for this damage multiplier is given by:

The maximum critical strike chance you can reach at level 1 through masteries is 20.25%, excluding abilities and items.

Critical strike chance has a hard cap at 100%.

Critical strike chance has a theoretical gold value of 50 gold per 1% critical strike chance. This makes Cloak of Agility the most efficient core item for critical strike chance, as it grants 15% critical chance for the cost of 730 gold with a 48.6 gold per % crit chance efficiency.

Champion abilities

(Note that any ability boosting attack speed has an indirect impact on the number of crits)

Ashe'sFocus gains 4 / 5 / 6 / 7 / 8 Focus stacks per second after not dealing damage in the last 3 seconds. At 100 stacks, Ashe will critically strike on her next basic attack. Thereafter, Focus stacks will reset to an amount equal to her critical strike chance.

Runes

Damage

Critical strike damage refers to the amount of damage that a champion's critical strike will deal. All champions have 200% critical strike damage by default, except Yasuo who has 180% to compensate for his doubled crit chance. In most cases, critical strike damage stacks additively. Damage modifiers apply to critical strike damage normally.